The invention relates to an optical disk apparatus with the use of an objective lens, and specifically relates to an optical disk apparatus capable of compatible reproduction (i.e. playback, read out) of optical disks whose substrate thicknesses are different. Hereinafter, the playback of contents in a disk and the read out of data in a disk are referred to as xe2x80x9creproduction.xe2x80x9d
In recent years, the optical disks have been improved in their capacity toward much higher recording density. So that now a DVD-ROM (Digital Video Disk Read Only Memory) having a 4.7 GB capacity is commercially available by contrast with a CD-ROM which is a household, read-only optical disk having 0.65 GB. As the most important difference between theses two media, the DVD and the CD have different thicknesses in their substrates, namely 0.6 mm for the DVD, 1.2 mm for the CD. This difference comes from a fact that the DVD uses a higher numerical aperture (NA) of 0.6, larger than that of the CD 0.45, with the aim at improving the recording density, which is and hence it is prerequisite in the DVD to suppress the occurrence of aberration arising from disk tilt etc. Here, the NA stands for an indicator of the objective lens to represent how much it can collect rays, and is defined as the sine of the vertex angle of the largest cone of meridional rays that can enter or leave an optical system or element, multiplied by the refractive index of the medium in which the vertex of the cone is located. Since it is essential for a DVD drive device to be capable of reproducing the CD disks, such difference of disk thickness presents a problem in choosing the xe2x80x9ccorresponding substrate thicknessxe2x80x9d of the objective lens.
First of all, the meaning of this correspondence substrate thickness will be described. In optical disks, in order to avoid a situation where a disk becomes not-readable for its recorded information due to dust etc., the disk is normally reproduced with a focused beam from and through the back side of its transparent substrate made from polycarbonate etc. whose front surface was processed to have a recording layer thereon. In this configuration, for example in the case of the DVD whose substrate thickness is 0.6 mm, the spot size on the back side of the substrate becomes 0.5 mm or so; therefore it is less likely that dust having a size of approximately 0.1 mm or less affect the reproduction. Because of this effect, unlike magnetic disks, the user can change the optical disks and reproduce those in person. However, in cases where light is focused through a substrate as described above, it is necessary to design the objective lens so as to have an appropriate lens shape in accordance with the substrate thickness. For example, if a transparent substrate having a certain thickness is inserted halfway in a bundle of rays that are converging ideally, there occurs a situation where a ray incident normally to the substrate and a ray incident obliquely thereto take different optical paths of different lengths: naturally the latter has a longer optical path than the former. Such optical path difference is called aberration, which gives rise to a cause that prevents sharp focusing. However, such aberration can be compensated by modifying the lens shape, assuming that the thickness of the substrate is to be inserted is known beforehand. Therefore, in optical heads of optical disk apparatuses, normally used are an objective lens that is made to match a thickness of the optical disk to be reproduced, after specifying the substrate thickness. As described above, the corresponding substrate thickness is defined as a substrate thickness such that when the substrate is inserted halfway in the bundle of convergent rays focused by a certain objective lens, that objective lens can focus the rays ideally without introducing aberrations.
Since normally each objective lens for optical disk has a unique corresponding substrate thickness, this fact became a problem when reproducing the DVD and the CD in a compatible manner.
To overcome this problem, for example, in the Japanese Published Unexamined Patent Application No. 6-124477, disclosed is a method where the numerical aperture of an objective lens for DVD is limited and hence rays passing through a peripheral part of the objective lens for DVD where the aberration is large are blocked, so that the aberration due to an error of the substrate thickness in the CD reproduction with the use of the lens for DVD is reduced. An aberration caused by the error of the substrate thickness is called a spherical aberration, which increases in proportion to the fourth power of NA. Consequently, if the NA is reduced with an aperture for a lens, the aberration of the lens is reduced accordingly. On the other hand, when reproducing the DVD, the lens is used without the aperture so that the light is not blocked thereby, being in a changeover manner. Here, this conventional example is intended to reproduce the CD by using light of a 650 nm wavelength which is a reproduction wavelength for the DVD. That is, the wavelength in this case is shorter than the 780 nm wavelength which is conventionally used for the CD. If a wavelength is shorter, a necessary NA can be decreased because the focused spot size is proportional to xcex/NA, where xcex denotes the wavelength. Therefore, the reproduction of the CD with the 650 nm wavelength light allows to reduce the NA of the aperture below 0.45 accordingly and thereby a necessary effect of reducing the aberration is achieved in the conventional example. However, the conventional example has a drawback that it cannot accommodate to the reproduction of the CD-R (CD-Recordable) that has a compatibility with the CD and is also a writable medium, currently becoming popular so much. This is because the CD-R uses a dye medium in its recording layer that has enough reflectance only at the 780 nm wavelength and the reflectance decreases at the 650 nm wavelength, namely the reproduction wavelength of the DVD, whereby a signal cannot be reproduced from the DVD. So it is necessary to mount a light source of the 780 nm wavelength separately to reproduce the CD-R. In this occasion, the wavelength becomes identical to that of a conventional optical head for CD and therefore it is necessary for the aperture limit NA to be set to no less than 0. 45, the value of the aperture limit NA for the conventional case, so that a sufficient effect of reducing the aberration can hardly be achieved by aperture limit.
Further, as another conventional example, in the Japanese Published Unexamined Patent Application No. 9-237431, disclosed is a method of blocking rays passing through a certain annular region of a lens. This example uses a method where, by blocking rays passing through the annular region where the aberration begins to change largely in the CD reproduction with the use of the objective lens for DVD, only the region that deteriorates directly the CD reproducing light spot is virtually removed. The reason for this configuration is that rays passing through a region that introduces a significantly large aberration consist of a wavefront of large inclination and each ray travels in a direction perpendicular to the wavefront, and consequently these rays disperse very largely from the focal position, so that substantially the rays do not affect the light spot quality by any means. In the DVD reproduction, rays passing through regions inside and outside the light-blocking annular region are focused virtually without the aberration. At this time, rays passing through the light-blocking annular region are blocked also in the DVD reproduction. By adjusting the width of that region so that loss in the light amount and deterioration of the light spot due to the light blocking are controlled to be within an allowance, virtually the same effect as that of the liquid crystal can be obtained without the liquid crystal. However, since this contrivance is virtually equivalent to the case of limiting the aperture, the method cannot be applied to the reproduction of the CD-R.
Furthermore, other prior art technology is described in the Japanese Published Unexamined Patent Application No. 8-55363. In this method, two kinds of optical disks having different thicknesses are reproduced by using light sources having different wavelengths. The two laser diodes are arranged with different distances set from the objective lens and the spherical aberration introduced by this particular arrangement is used to compensate the spherical aberration introduced by the error of the substrate thickness. With this method, the DVD and the CD can be reproduced with a single objective lens in a compatible manner while a 780 nm wavelength laser diode is used for the reproduction of the CD and a 650 nm wavelength laser diode is used for the reproduction of the DVD, and additionally the CD-R can be reproduced. However, it is known that when two light sources are arranged at different positions like this, the tolerance of relative displacement between the laser diode and the objective lens becomes narrower. This is because there exists a so-called sine condition that prescribes a condition in a design of an objective lens wherein, even when the incident angle of the incident light deviates from the proper angle, the coma aberration does not increase rapidly and this condition can stand only for a specific object-image distance. Therefore it cannot stand for two kinds of the object-image distances. That is, if the arrangement of the 650 nm wavelength laser diode satisfies the sine condition, that of the 780 nm wavelength laser diode cannot satisfy this condition.
In such a case, for example, if the disk rotates eccentrically, the objective lens is made to move in tracking the track of the rotating disk, and in return this movement breaks the relative positional relation between the objective lens and the laser diodes, hence increasing the aberration very rapidly.
Yet furthermore described is a prior art technology, for example, in the Japanese Published Unexamined Patent Application applied for by the present inventors et al. In FIG. 1 of this application, fabricated is an objective lens 1 that has the corresponding substrate thickness of 0.76 mm in an inner region 11 and that of 0.6 mm in an outer region 12 and is further provided with the annular phase shifter 13 consisting of a recessed annular groove. The corresponding substrate thickness of the phase-shifting annular region is set to 0.76 mm, the same value as that of the inner region 11. In this objective lens 1, in order to mitigate the difference between the corresponding substrate thickness for the CD and that for the DVD, the corresponding substrate thickness in the inner region 11 plus a region of the annular phase shifter 13 is set to 0.76 mm, which is a compromise value for 0.6 mm for the DVD, with an intention to get close to 1.2 mm for the CD to effect the reduction of the aberration introduced in the inner region in the CD reproduction; and further, in order to reduce residual aberration and to reduce also the aberration introduced due to the error of the substrate thickness in the DVD reproduction, the annular phase shifter 13 is employed. By the way, in the above-mentioned conventional example where either one of two beams of different wavelengths entering the objective lens is not a parallel beam, there occurs a problem that the aberration of that beam increases abruptly when the objective lens is displaced in a direction perpendicular to its optical axis by the tracking. On the contrary, both the 650 nm wavelength light and the 780 nm wavelength light are intended to enter this objective lens as parallel beams; therefore this configuration has a merit of being free from the above-mentioned problem.
FIG. 2 is a diagram showing the calculated wavefront aberration profiles in the DVD reproduction and in the CD reproduction in the conventional example. Here, the wavefront aberration is represented as the deviation of optical path length from a laser diode serving as a light source to the focus of an object lens as a function of a ray position in the bundle of effective rays (pupil). However, in FIG. 2, the aberration exceeding a range of xe2x88x920.5xcex to +0.5xcex is reduced to a value within the range, which has virtually the same optical effect as that of the value before reduction. The phase shift of the annular phase shifter is assumed to be xe2x88x921.7xcex in the CD reproduction. Here, a negative phase shift means shortening of the optical path and the step causing such is a recess. Then, a positive phase shift of 0.24 X, which is obtained by reducing the amount of xe2x88x921.76xcex to a value within the range of xe2x88x920.5xcex, to +0.5xcex through the addition of 2xcex, is virtually introduced in the CD reproduction. On the other hand, the introduced aberration becomes xe2x88x921.76xcex/(1.583xe2x88x921).times. 780/650. times. (1.586xe2x88x921)=xe2x88x922.12xcex in the DVD reproduction, assuming the refractive index at 780 nm to be 1.583 and that at 650 nm to be 1.586, respectively. Then, in the same manner as mentioned above, a negative phase shift of xe2x88x920.12xcex, which is obtained by reducing xe2x88x922.12xcex to a value within a range of xe2x88x920.5xcex to +0.5xcex through the addition of 2xcex is virtually introduced. Since the sign of the phase shift required at each wavelength is previously determined, the effect like this is not equivalent for the case where the annular phase shifter is a recess and for the case where it is a protrusion. For example, here we will consider how much phase shift is required when using a protruding phase shifter. Assuming that the phase shift in the CD reproduction is +3.24xcex, the necessary phase shift is calculated as 3.24/(1.583xe2x88x921).times.780/650.times.(1.586xe2x88x921)=3.91xcex, which is reduced to xe2x88x920.09xcex by subtracting 4 2. therefrom so as to find a value within the range of xe2x88x920.5xcex to +0.5xcex. However, when the absolute value of the original phase shift including an integer part becomes large as in the above case, the fabrication of the lens becomes difficult not only because the step to be formed becomes deeper but also because the phase shift error arising from the deviation of the wavelength caused by the temperature variation etc. will increase almost in proportion to the integer part (2n7xcfx80) of the original phase shift. Therefore, it is preferable that the step is a recess in the design. Regarding the wavefront aberration obtained in the above procedure as shown in FIG. 2, in order to focus the light spot excellently, it is necessary for its RMS value to be, for example, 0.07xcex or less (xcex being a light wavelength) according to the Marechal""s criterion. On the contrary, in the DVD reproduction, the RMS wavefront aberration becomes 0.02xcex, indicating excellent performance. In the CD reproduction, although the wavefront aberration is significantly large at a peripheral part of the lens, the wavefront aberration in the range of an effective NA by design satisfies the above-mentioned criterion. The rays passing through the peripheral part of the lens suffer an increasingly large aberration; therefore a wavefront consisting of these rays has a large angle of inclination. Consequently such rays disperse easily and do not affect the light spot composed of virtually the rays passing through the central part of the lens by any means. In this way, the compatibility in the reproduction of the DVD, CD, and CD-R can be realized with this design.
However, in the conventional example mentioned above where the annular phase shifter is integrated with the objective lens to form a single-piece body, there is a problem that the lens is originally designed not for the recording of the CD-R but only for the reproduction of the CD-R; therefore the reduction of the aberration is not sufficient as a CD-R recording device. This problem came from a fact that it is necessary to use an objective lens having an NA of 0.5 which is larger than the NA necessary for the reproduction of the CD in order to form recording marks comparable to recorded information pits of the CD with a high degree of accuracy in a CD-R recording device. The conventional example mentioned above is intended to perform only the reproduction of the CD-R in addition to the reproduction of the CD-ROM, hence beings capable of only focusing a light spot equivalent to that of the CD-ROM optical pick-up having an NA of 0.45 at the 780 nm wavelength. Therefore in the range of an NA of 0.5, the objective lens has an increased aberration, so that the objective lens cannot cope with the recording of the CD-R where an NA of 0.5 is indispensable.
In consideration of the above-mentioned problems, it is the object of the present invention and a problem to be solved thereby to provide an optical disk apparatus that uses an objective lens capable of reproducing the DVD and recording the CD-R in a compatible manner.
To solve the above-mentioned problem, in the objective lens having at least two refraction planes according to the above-mentioned conventional example, a circular region in the vicinity the optical axis of the lens is protruded so as to provide a step at least on one plane of the two refraction planes.
That is, a circular phase shifter whose phase shift direction is reversed to that of the annular phase shifter is provided additionally on the objective lens. With this modification, the unevenness of the wavefront aberration profile in the central region in FIG. 2 can be mitigated, hence reducing the RMS wavefront aberration, and the diameter of the bundle of effective rays in the CD reproduction can be increased, hence achieving a lens performance of focusing a light spot necessary for the recording of the CD-R.
Further, in modifying the lens in this manner, as described in the conventional example mentioned above, at least this phase-shifting circular region is designed to have an optimum substrate thickness closer to 1.2 mm of the CD substrate thickness rather than 0.6 mm of the DVD substrate thickness. In other words, as far as only this region is concerned, the aberration when reproducing the optical disk whose substrate thickness is more than 0.6 mm becomes smaller than that when reproducing the optical disk whose substrate thickness is 0.6 mm. Moreover, at the same time, an outer region beyond the annular region is designed to have an optimum substrate thickness of 0.6 mm so as to be optimized for the DVD. That is, this outer region has a shape such that the aberration is minimized when reproducing the optical disk whose substrate thickness is 0.6 mm.
Specifically, this particular design becomes most effective, denoting the ratio of the diameter of the above-mentioned circular region to that of the bundle of total effective rays as D1, a ratio of the diameter of the inner rim of the above-mentioned annular region to that of the bundle of total effective rays as D2, and a ratio of the diameter of the outer rim of above-mentioned annular region to that of the bundle of total effective rays as D3, when these parameter are set to satisfy the following conditional expressions:
0.22xe2x89xa6D1xe2x89xa60.35, (expression)
0.40xe2x89xa6D2xe2x89xa60.50, (expression)
0.75xe2x89xa6D3xe2x89xa60.82, (expression)
D1xe2x89xa6D2xe2x89xa6D3. (expression)
Further, in this objective lens, the effect of reducing the aberration becomes larger when the substrate thickness that introduces the smallest aberration at least in rays passing through above-mentioned circular region is set between 0.74 mm and 0.85 mm. Further, an optical head capable of reproducing the DVD and recording the CD-R in a compatible manner can be realized only by integrating this objective lens with at least two laser diodes emitting different wavelength beams, a lens, a mirror, a photodetector, etc. to construct an optical head in a single structural combination.
Further, in the objective lens to be used in such an optical head furnished with two wavelength light sources like these, denoting optical path differences generated by the steps in the above-mentioned circular region and in the annular region as xcfx861 and xcfx862, respectively, and the longer wavelength of the two wavelengths, namely the wavelength for the reproduction of the CD, as xcex1, the performance thereof is enhanced most effectively when these parameters are set to satisfy the following conditional expressions:
1.67xcex1xe2x89xa6xcfx861xe2x89xa61.87xcex1, (expression)
xe2x88x921.87xcex1xe2x89xa6xcfx862xe2x89xa6xe2x88x921.67xcex1. (expression)
In this case, denoting the shorter wavelength of the two wavelengths, namely the wavelength for the reproduction of the DVD, as xcex2, xcfx861 and xcfx862 are assured to satisfy the following conditional expressions at the same time:
2.00xcex2xe2x89xa6xcfx861xe2x89xa62.24xcex2, (expression)
xe2x88x922.24xcex2xe2x89xa6xcfx862xe2x89xa6xe2x88x922.00xcex2. (expression)
Further, an optical head capable of reproducing the DVD and recording the CD-R in a compatible manner can be realized only by integrating this objective lens comprising these steps with at least two laser diodes emitting different wavelength beams, a lens, a mirror, a photodetector, etc. to construct an optical head in a single structural combination.
Further, an optical disk apparatus capable of reproducing the DVD and recording the CD-R in a compatible manner can be realized only by integrating this optical head with a mechanism, a control circuit, etc. all of which serves to perform the reproducing or the recording/reproducing of at least two kinds of optical disks having different substrate thicknesses to construct an optical disk apparatus in a single structural combination.